Malignant gliomas are the most common primary brain tumour and are associated with a very poor prognosis (Wrensch et al, 2002). It has been hypothesised that gliomas arise from endogeneous glial progenitor or neural stein cells (Canoll and Goldman, 2008), with which they share the ability to migrate along white matter tracts and perivascular and subpial spaces (Louis, 2006). As a consequence, malignant gliomas are highly infiltrative tumours for which complete surgical resection is not feasible. The limitations of conventional treatment modalities at adequately treating infiltrative tumour cells are highlighted by the observation that 80% of malignant gliomas recur within 2 to 3 cm of the original tumour mass (Hess et al, 1994).
Herpes Simplex Virus (HSV-1) is a large, naturally neurotropic, double-stranded DNA virus that is actively being developed into useful replication-selective (oncolytic) and replication-defective gene therapy vectors (Bowers et al, 2003). To date, two replication-selective viral constructs have reached clinical trials in patients with malignant gliomas (Rampling et al, 2000; Marken et al, 2000; Papanastassiou et al, 2002; Harrow et al, 2004). These viruses, designated G207 and HSV1716, harbour null mutations in both copies of the γ134.5 gene. The products of this gene are critical in enhancing the ability of HSV-1 to infect neurones and overcome host cell responses to viral infection (He et al, 1997). In addition, null mutations of the γ134.5 gene confer the ability of these vectors to selectively replicate in tumour cells (Shah et al, 2003).
To date, in all clinical trials of selectively-replicating HSV-1, vector administration has been achieved by direct intratumoural or intraparenchymal injection. Early clinical trials involved the direct injection of vector directly into the MRI-enhancing tumour mass (Rampling et al, 2000; Marken et al, 2000; Papanastassiou et al, 2002). These studies demonstrated safety and provided limited evidence of in vivo replication of HSV1716 in patients with malignant gliomas. However, conclusive evidence of significant vector distribution and treatment efficacy has yet to be demonstrated. Although this may relate to methodological difficulties of confirming vector replication clinically, there is significant uncertainty regarding the effectiveness of intratumoural injection (Dempsey et al, 2006).
By definition Grade IV gliomas are characterised by areas of tissue necrosis (World Health Organisation, 2007). Consequently the direct inoculation of a necrotic primary tumour mass with a replication-selective viral vector capable of replicating within live malignant glioma cells is unlikely to efficiently treat either the primary tumour mass or more importantly, the infiltrating tumour cells. In addition, the primary tumour mass is often amenable to surgical resection rendering intratumoural injection of replication-selective vector unnecessary. Indeed, Harrow et al (2004) undertook a phase I/II study of peri-tumoural injections of HSV1716 in patients undergoing resection of either recurrent or newly diagnosed malignant gliomas. This study demonstrated this approach to be safe, although it is clearly critical that for this approach to be efficacious, viral distribution must be optimised to facilitate the transduction of as many infiltrating tumour cells as possible.
Convection-enhanced delivery (CED) involves the use of fine catheters and precisely controlled infusion rates to distribute therapeutic agents by bulk-flow directly into the brain extracellular space, possibly along the same extracellular pathways that glioma cells are able to migrate. In contrast to techniques of drug delivery that depend on diffusion to achieve adequate drug distribution, such as carmustine-impregnated biodegradable polymers, with CED it is possible to distribute drugs homogeneously over potentially large volumes of brain, irrespective of the molecular size of the therapeutic agent (Morrison et al, 1994). As such it is an ideal technique for the administration of viral vector-mediated gene therapy to the brain of patients with malignant gliomas.
HSV-1 vectors have a diameter of 120 to 300 nm (Jacobs et al, 1999), whereas on average the brain extracellular space has a diameter of 38 to 64 nm (Thorne and Nicholson, 2006). Clearly this has the potential to make the administration of HSV-1-based vectors by CED unachievable. Consequently, in this study the distribution of a replication-selective HSV-1 viral construct by CED has been examined in both grey and white matter and, a variety of strategies to enhance viral vector distribution have been evaluated.
Nevertheless, in addition to the aforementioned clinical trials (6-9), HSV vectors have been administered by stereotactic injection into normal mouse (17-19), rat (20-26) and primate brains (20-28), animal models of high-grade glioma (29-35), mucopolysaccharidosis type VII(36), GM2 gangliosidosis (37) and Parkinson's disease (37-39), as well as being administered by CED into a glioma rat model (40). In view of this large number of studies it is surprising that to date no attempt has been made to systematically evaluate and optimise the delivery of these vectors directly into the brain. Consequently, in this study the distribution of a replication-selective HSV-1 viral construct by CED has been examined in both grey and white matter and, a variety of strategies to enhance viral vector distribution have been evaluated.